Bisphenols in the form of condensation products of phenols and carbonyl compounds are source materials or intermediate products for the manufacture of a large number of commercial products. The condensation product arising from the reaction between phenol and acetone, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A, BPA), is of particular technical importance. BPA serves as source material for the production of diverse polymeric materials, such as, for example, polyarylates, polyether imides, polysulfones and modified phenol-formaldehyde resins. Preferred fields of application lie in the production of epoxy resins and polycarbonates.
Technically relevant methods for producing BPA are known and are based on the acid-catalysed conversion of phenol with acetone, wherein a phenol/acetone ratio of more than 5:1 in the reaction is preferably adjusted. Homogeneous and also heterogeneous Bronsted acids or Lewis acids may be used as acidic catalysts, for instance strong mineral acids such as hydrochloric acid or sulfuric acid. Gel-like or macroporous sulfonated cross-linked polystyrene resins (acidic ion-exchangers) are preferably used. By way of cross-linker, divinylbenzene is normally employed, but others, such as divinylbiphenyl, may also find application. In addition to the catalyst, use may be made of a co-catalyst. In this connection thiols that carry at least one SH function may be used. The co-catalyst may be either dissolved homogeneously in the reaction solution or, in the case of the acidic ion-exchangers, fixed to the catalyst itself. Homogeneous co-catalysts are, for example, mercaptopropionic acid, hydrogen sulfide, alkyl sulfides such as, for example, ethyl sulfide and similar compounds. Fixed co-catalysts are aminoalkyl thiols and pyridylalkyl thiols which are ionically bonded to the catalyst, in which case the SH function may be protected and is only set free during or after fixing to the catalyst. Similarly, the co-catalyst may be covalently bonded to the catalyst in the form of alkyl thiol or aryl thiol.
In the course of the conversion of phenol with acetone in the presence of acidic catalysts a product mixture results which, besides unconverted phenol and, where appropriate, acetone, primarily contains BPA and water. In addition to these, typical by-products of the condensation reaction arise in small quantities, for instance 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl)propane (o,p-BPA), substituted indanes, hydroxyphenyl indanols, hydroxyphenyl chromanes, substituted xanthenes and more highly condensed compounds with three or more phenyl rings in the molecular skeleton. In addition, further secondary components such as anisole, mesityl oxide, mesitylene and diacetone alcohol may be formed as a result of self-condensation of the acetone and reaction with impurities in the raw materials.
For economic and technical reasons, the reaction is usually conducted in such a way that one-hundred-percent conversion of the acetone is not obtained, and 0.1–0.6 wt. % is still contained in the outflow of the reactor.
The named by-products, such as water and unconverted feed materials phenol and acetone, impair the suitability of BPA for producing polymers and have to be separated by suitable processes. High purity demands are made of the raw material BPA, particularly for the purpose of producing polycarbonate.
One method for reprocessing and purifying BPA is undertaken by separation of BPA from the reaction mixture in the form of a roughly equimolar crystalline adduct with phenol by cooling of the reaction mixture accompanied by crystallization of the BPA/phenol adduct in a suspension crystallization. The BPA/phenol adduct crystals are then separated from the liquid phase by means of a suitable apparatus for solid/liquid separation, such as rotary filters or centrifuges, and are supplied to a further purification stage.
Adduct crystals obtained in this way typically have a purity of >99 wt. % BPA, relative to the sum of BPA and the secondary components, in the case of a phenol proportion of approx. 40 wt. %. By washing with suitable solvents, which typically contain one or more components from the group comprising acetone, water, phenol, BPA and secondary components, the adduct crystals can be freed of superficially adhering impurities.
The flow of liquid (mother liquor) resulting in the course of the solid/liquid separation contains phenol, BPA, water that has arisen in the course of the reaction, unconverted acetone, and is enriched with the secondary components typically resulting in the course of the production of BPA. This flow of mother liquor is conventionally recirculated into the reaction unit. In order to maintain the catalytic activity of the acidic ion-exchangers, resulting water is removed by distillation, whereby acetone that is still present in the given case is also removed from the mother liquor. The dewatered reaction flow which is obtained in this way is replenished with phenol and acetone and is recirculated into the reaction unit. Alternatively, water and acetone may also be removed by means of distillation prior to carrying out the suspension crystallization of the BPA/phenol adduct. In the course of the stated distillation steps, a partial quantity of the phenol that is present in the reaction solution may also be removed by means of distillation. EP-A-1 162 188 describes such a separation of the reaction water and of the incompletely reacted acetone by means of distillation.
With such a circulatory mode of operation the problem arises that by-products of the production of BPA are enriched in the circulatory flow and result in the deactivation of the catalyst system and also in inferior product quality. In order to avoid an excessive enrichment of secondary components in the circulatory flow, a partial quantity of the circulatory flow—optionally after partial or complete recovery of phenol by distillation—is extracted from the process chain in the form of so-called BPA resin.
In addition, it has proved advantageous to conduct a fraction or the total quantity of the circulatory flow, after the solid/liquid separation and before or after the separation of water and residual acetone, across a rearrangement unit filled with acidic ion-exchanger. This unit is generally operated at higher temperatures than the reaction unit. In this rearrangement unit some of the secondary components of the production of BPA which are present in the circulatory flow are isomerized under the prevailing conditions to form BPA, so that the overall yield of BPA may be increased.
The BPA/phenol adduct crystals obtained subsequent to the suspension crystallization of the reaction solution, described above, and the solid/liquid separation are supplied to ongoing purification steps, in the course of which the separation of phenol and optionally the lessening of the concentration of secondary components are achieved.
In this way, the BPA/phenol adduct crystals may be recrystallised from phenol, organic solvents, water or mixtures of the named solvents, which optionally still contain BPA and/or its isomers, by a suspension crystallisation. In this connection, the phenol that is present in the adduct crystals can also be separated entirely or partially through the choice of suitable solvents. The phenol remaining in the BPA in the given case after the recrystallisation is then separated entirely by suitable distillative, desorptive or extractive methods.
Alternatively, the phenol may also be removed from the BPA/phenol adduct crystals by means of melting-out processes. But with these processes the BPA is subjected to thermal loads, resulting in undesirable dissociations of BPA.
After the separation of phenol, a bisphenol-A melt is obtained which may be used without prior solidification for the production of polycarbonate by the transesterification process (molten polycarbonate). But the bisphenol-A melt may also be solidified, for sale or for alternate usage, by known processes, such as, for example, by the prilling process or by exfoliation. Furthermore, the melt may be dissolved in caustic-soda solution and may be employed in the polycarbonate process in accordance with the interphase process. The bisphenol A that has been freed of phenol may optionally be subjected, prior to further processing, to a purification step such as, for example, a melt crystallization, a distillation and/or a recrystallization out of phenol, water or an organic solvent such as, for example, toluene or mixtures of these substances.
Prior to being recirculated into the reaction, the mother liquor has to be dewatered, since by reason of the thermodynamic equilibrium the reaction water would greatly suppress the reaction of phenol and acetone to form bisphenol A and would consequently reduce the conversion arising from the reaction. On the other hand, the unconverted residual acetone should remain in the reaction solution, since otherwise it would have to be isolated in elaborate manner from side flows and would have to be recirculated into the reaction in order to keep losses of raw materials down for economic reasons.
The object of the present invention was therefore to make available a process for producing bisphenol A with which the reaction water may be removed from the reaction solution or from the mother liquor that stems from the crystallization and filtration, whereby the unconverted residual acetone remains largely in the reaction solution.